Photolithography is a well known technique used to apply patterns on the surface of a workpiece, such as a circuit pattern to a semiconductor chip or wafer, and which is also capable of producing very small, intricate patterns for many other applications. Traditional photolithography involves applying electromagnetic radiation to a mask having openings formed therein such that the light or radiation that passes through the openings is applied to a region on the surface of the workpiece. The surface regions are referred to herein as "electromagnetic radiation application regions".
The workpiece surface is typically coated with a photoresist such that the electromagnetic radiation application regions (hereinafter referred to as "EAR"s) where electromagnetic radiation is applied through the mask are selectively exposed or selectively left unexposed depending upon the properties of the photoresist. For a positive photoresist, regions which have electromagnetic radiation applied dissolve upon application of a developer solution, while regions which are not exposed do not dissolve in the developer. With negative photoresists the reverse occurs.
Photolithographic resolution is an important factor in determining a minimum dimension between openings in a mask which will provide unaffected spaces of a desired dimension between the EARs on the surface. When two adjacent or juxta-posed mask openings are too close, since the electromagnetic radiation travelling through each of the adjacent openings is diffracted the resulting fringes can cause constructive and destructive interference. This diffraction can diminish a dimension of the unaffected spaces formed on the workpiece surface, and under some circumstances may increase the intensity of the light radiation within these unaffected spaces.
As new generations of photolithographic techniques develop, the minimum dimensions of the EARs formed on the workpiece surface by photolithography become smaller, as do the minimum dimensions of the unaffected spaces between the EARs. In other words, the minimum dimensions of circuits formed by photolithographic processes generally decrease as improvements in semiconductor manufacturing processes occur. Improving photolithography technology provides improved resolution, resulting in a potential reduction of the minimum dimensions of EARs and the spaces between EARs. However, the process of developing and constructing masks capable of providing a higher resolution, as well as the development of the associated mask-making equipment, is expensive. It is desirable that the masking techniques associated with semiconductor chip manufacturing be able to reliably provide minimum sized unaffected spaces of smaller dimensions. In fact this has been, and still is, a highly desirable goal of photolithography in general.
Recent improvements in photolithographic masks often involve phase shifting techniques, in which certain of the openings, or portions of openings, are phase shifted with respect to adjacent openings. The openings in the mask are typically configured in an array of openings which are alternately phase shifting, and non-phase shifting, along two perpendicular axes of the mask. The electromagnetic radiation that passes through the phase shifting openings interferes destructively in the spaces with the electromagnetic radiation passing through the non-shifting openings, and thereby reduces the intensity of electromagnetic radiation within the unaffected spaces as described below. Recent improvements enable EARs and unaffected spaces to be formed with smaller dimensions. A related article, "Improving Resolution in Photolithography with Phase Shifting Masks", by Levenson et al., in IEEE Transactions on Electron Devices, Vol. ED-29, No. 12--December (1982), describes the photolithography process in general as well as the above-described specific form of photolithography. The specific type of photolithography described in the Levenson article is referred to in the present disclosure as "alternating phase shifting" photolithography.
Any mask configuration wherein any adjacent pair of EARs are spaced such a distance from each other as to permit electromagnetic radiation passing through the adjacent openings from interfering destructively with each other is suitable for phase shifting techniques. Any location on a mask where openings which are located closely adjacent to each other, and which are not correctly phase shifted relative to each other, is referred to herein as a conflict. Any mask with a conflict produces EARs with a poorly defined unaffected space therebetween.
Another technique which limits conflicts in photolithography is known as the orthogonal separation (hereinafter referred to as "OSEP") technique. OSEP limits many conflicts between two or more mask openings which extend perpendicular to each other. In OSEP techniques, all of the openings which extend primarily in a first direction are referred to as X component openings of the mask. The openings which extend primarily in a second direction, which is perpendicular to the first direction, are referred to as Y component openings of the mask. Since each X component opening extends primarily along the X direction, the possibility that an X component opening will interfere with a Y component opening is much higher than the possibility that the X component opening will interfere with another X component opening. The converse of the last sentence is also true. Therefore, the OSEP technique utilizes two masks defined as an X component mask and a Y component mask. The X component mask contains all, or almost all, of the X component openings. The Y component mask contains all, or almost all, of the Y component openings. The OSEP technique thereby limits the occurrences of conflict in complex mask layouts between X component openings and Y component openings. However, there are still some mask layout configurations which result in conflicts when OSEP techniques are used.
Alternating phase shifting techniques, as well as other types of phase shifting techniques which are known in the art, do improve the resolution of photolithographic masks in many layout configurations. The alternating phase shifting process functions best when the opening pattern formed on each mask is regular and repeatable. However, when the openings are staggered and irregular, it often becomes difficult to provide a mask layout which ensures that each opening is alternately phase shifted without conflicts with respect to each of its adjacent openings.
Several problems still have to be overcome to make the alternating phase shifting mask technique viable for use with arbitrary patterns. The first problem is that alternating phase shifting using light field masks often produces unwanted opaque images due to uncovered phase shifting open edges. The adverse effects of opaque image edges can, however, be limited by using only dark field masks with the resist polarity selected to suit the masks being used. The second problem is that when the patterns are unevenly spaced, certain adjacent phase shifting openings have different separations providing different levels of phase shifting improvement. The second problem can be overcome by combining alternating phase shifting technology with rim (Rim) techniques, or using attenuated (Att) and unattenuated (Utt) phase shifting techniques. Rim techniques are used where the openings are isolated far enough from each other so that conflicts between electromagnetic radiation passing through each opening is not a concern. The electromagnetic radiation passing through an opening adjacent each edge of the opening is phase shifted with respect to the electromagnetic radiation passing through the center of the opening. This configuration provides destructive interference for the electromagnetic radiation which strikes the surface at a region outside of where the electromagnetic radiation is intended to be directed. This destructive interference occurs between the electromagnetic radiation passing through the edges of the opening, and the electromagnetic radiation passing through the center of the opening. Rim technology further enhances the resolution of photolithography when openings are distantly spaced from each other.
The third problem with arbitrary two dimensional pattern layouts is that the alternating phase shifting process is not self-consistent. While the above described alternating phase shifting technology improves simpler mask layouts in general, it is desirable to make the alternating phase shifting technology applicable to masks with more complex and arbitrary layouts. It is desirable to apply alternating phase shifting masks in situations where a pair of laterally spaced overlapping openings overlap both of two or more longitudinally spaced openings as exemplified in FIG. 3a.
It would be highly desirable to provide some technique by which the alternating phase shifting processes can be applied in those instances where the opening configurations are overlapped, staggered or irregular. It is also desirable to limit conflicts in as many instances as possible. Accomplishing these features in general would improve the overall application of alternating phase shifting masks. It would also permit the application of more alternating phase shifting techniques to more complex masks.